Motors and other electromagnetic components are often used in alternating magnetic fields and employ magnetic cores (core materials). Such magnetic cores have been produced by stacking electromagnetic steel sheets to give a laminate and processing the resulting laminate. The magnetic cores obtained by processing electromagnetic steel sheets are, however, magnetically anisotropic, and this impedes designing of electromagnetic components having three-dimensional magnetic circuits. To avoid this, production of dust cores by compacting an iron-based soft magnetic powder has been recently investigated. This is because such dust cores are magnetically isotropic and enable designing of electromagnetic components having three-dimensional magnetic circuits.
Production of dust cores employs a powder including an iron-based soft magnetic powder covered with an insulating coating. Coverage of an iron-based soft magnetic powder with an insulating coating suppresses the generation of an inter-granular eddy current and thereby gives a dust core with a lower eddy current loss. However, the iron-based soft magnetic powder covered with the insulating coating disadvantageously gives a dust core having a high coercive force, a large hysteresis loss, and insufficient magnetic properties, because interfaces between the coated powder particles impede the flow of magnetic flux.
Techniques for reducing the coercive force to improve magnetic properties of dust cores can be found typically in Japanese Unexamined Patent Application Publication (JP-A) No. H03-223401, JP-A No. 2011-114321, and JP-A No. 2006-302958.
Specifically, JP-A No. H03-223401 mentions that a magnetic card is coated with a coating including a fine powder of a high-permeability material for the purpose of magnetic shielding and that the coating powder should have a high magnetic permeability, be a fine powder, and have a flattened shape. However, such a flattened powder, when compacted, is orientated, and this adversely affects the advantage of dust cores, i.e., magnetic isotropy.
JP-A No. 2006-302958 mentions that a specific soft magnetic material can give a compact having higher strengths with a lower eddy current loss, which soft magnetic material has a ratio of a maximum diameter to an equivalent circle diameter of more than 1.0 and equal to or less than 1.3 and has a specific surface area of 0.10 m2/g or more. This literature also mentions that a water-atomized powder has a large number of projections on the surface and, when it is used as metal magnetic particles, the surface of the water-atomized powder is worn out with a ball mill to remove the projections.
JP-A No. 2011-114321 discloses soft magnetic particles having a degree of sphericity of 0.9 or more, a coercive force of 500 Oe or less, and an apparent density of 1.6 g/cm3 or more. This literature mentions that soft magnetic particles, when suitably controlled on degree of sphericity, coercive force, and apparent density and when used as a material for a dust core, gives a dust core which has a lower hysteresis loss and a lower eddy current loss and exhibits high strengths. The literature also mentions that soft magnetic particles are spheroidized by molding a material of the soft magnetic particles into pellets, firing the pellets, pulverizing the burned product, and supplying the pulverized product into flame to melt the pulverized product in a suspending state to thereby form spherical particles.
However, the techniques disclosed in JP-A No. 2006-302958 and JP-A No. 2011-114321 require a granulation step for spheroidizing a soft magnetic material and thereby fail to reduce production cost.